Single-piece endoprosthesis with high expansion ratios and atraumatic ends

ABSTRACT

A new endoluminal prosthesis for use in sealing a vascular graft to corporeal lumen provides for a flattened bulbous tail at the end of each cell of the prosthesis. The flattened bulbous tails reduce the amount of wear between the prosthesis and the softer material of the vascular grafts or corporeal lumen walls. A method of manufacturing a stent from a flat sheet of material is also included.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application U.S. Ser. No. 11/110,999filed on Apr. 20, 2005 now U.S. Pat. No. 7,740,653 which is a divisionalapplication of Ser. No. 10/090,473 filed on Mar. 4, 2002 which issued onSep. 13, 2005 as U.S. Pat. No. 6,942,690; which is acontinuation-in-part application of Ser. No. 09/837,353 filed on Apr.17, 2001 now abandoned; which is continuation-in-part application ofU.S. Ser. No. 09/546,966, filed on Apr. 11, 2000 now abandoned; whosecontents are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates to medical devices for the treatment of vasculardiseases generally referred to as endoluminal prostheses. A variety ofsuch devices are available for a broad range of treatment modalities.Examples of such devices are “vascular grafts” and “stents.” Vasculargrafts are typically used to treat weakened areas of vessels known asaneurysms. Stents are typically used to prop open a narrowed or stenosedvessel.

Stents and grafts may be delivered intraluminally through a narrowincision or a puncture in the patient's skin. The device may be mountedon a delivery catheter and inserted into a corporeal lumen through theskin. The device and catheter are then advanced through the variouslumens to the site to be treated. To accomplish this, stents and graftsare generally collapsible for delivery and expansible for treatment.

Vascular grafts are primarily composed of an artificial lumen whichisolates the natural lumen from the flow of bodily fluids, such asblood. Grafts may incorporate attachment devices to secure the graftinto the natural lumen and keep the graft expanded. Stents are typicallyformed of metallic wires or bars configured in a cylinder. Prior artstents for sealing a graft to corporeal lumen include narrow, sharptails that can cause wear to the relatively soft graft. The wear is afunction of the radial force of the stent, the sharpness of the stent,and the amount of relative motion between the stent and the graft.

The prior art also teaches methods of manufacturing stents from tubularshape material. Such methods require the manufacturer to remove materialfrom radial surfaces to produce the stent pattern. Raw tubular shapematerial, as well as processing tubular shape material, is relativelymore expensive than the cost and processing of the material in otherforms.

Hence, those skilled in the art have recognized a need for providing aprosthesis which produces less wear between the prosthesis and thesofter graft material. The need producing stents with non-tubular shapematerials has also been recognized. The present invention fulfils theseneeds and others.

SUMMARY OF THE INVENTION

Briefly and in general terms, the present invention relates to animproved endoluminal prosthesis. This prosthesis may function as a stentor as a means to secure an endoluminal graft in a corporeal lumen suchas an artery. The stent may include flattened bulbous tails to reducethe amount of wear between the stent and the softer graft material orcorporeal lumen tissue. Stents typically are used to ensure the patencyof diseased corporeal lumens by resisting collapse and occlusion.Endoluminal grafts typically are used to isolate diseased corporeallumens from the flow of bodily fluids. The present invention alsorelates to a method of producing a tubular stent from a flat sheet ofmaterial.

The prosthesis incorporating the present invention is configured as aseries of intermittently merging curved beams (e.g. leaf springs) formedinto a cylinder. This cylindrical structure is capable of beingcompressed into a small diameter and expanded into a large diameter. Tofacilitate both compression and expansion the beams have a cross-sectionwhich is greater in the radial direction (thickness) than in thecircumferential direction (width). The beams of the present inventionare also generally continuously curved to reduce or minimize stressconcentrations in the structure. The beams straighten during compressionuntil they are nearly straight.

While compressed the thickness of the beams prevents overlap. In atightly packed configuration, the curved beams straighten out, cometogether and generally lie flat in close proximity to each other. Thebeams resist overlap because the thickness of each beam requiressubstantial radial displacement to move over or under the adjacent beam.The compression of the prosthesis may be maximized by configuring thebeams to fit together tightly in a collapsed condition.

While expanded and during expansion, the thickness of the beams and theconfiguration of the beams increase the strength of the prosthesis andreduce or minimize stress concentrations. Thicker beams provide for morematerial in the radial direction to prevent radial collapse. The curvedconfiguration of the beams spreads the bending due to expansionthroughout the entire length of the beam. This prevents one area of thebeam from generating most of the bending and withstanding resultantstress concentrations.

In various preferred embodiments of the present invention, furtherimprovements distribute stresses throughout the beams more evenly. Forexample, the extreme ends of adjacent beams may be connected by a loopor eyelet connector. In such an embodiment the stresses from bending dueto compression of the prosthesis concentrate in the loop portion of theconnector until the lower portion of the connector just adjacent to theloop portion closes on itself, bringing the adjacent beams into contact.Further compression after that point concentrates stresses in the beambelow the loop. A similar result can be achieved by configuring thebeams to form a significant area of contact adjacent other types ofconnectors prior to full compression of the prosthesis.

The present invention is a single integrated structure without welds orfasteners. This may be accomplished by removing almond-shaped cells froma thick-walled cylinder. This eliminates the need to construct theprosthesis from individual pieces and possible weak points created byfasteners or joining.

In a first embodiment, the prosthesis may have curved beams which areonly merged to adjacent beams at their end points. This creates a singlerepetitive pattern around the circumference of the cylinder, with eachbeam merged to opposite adjacent beams at opposite end points. Thisembodiment may be viewed as the simplest structure to include theinvention described herein. It includes alternating half-cells dividedby curved beams. This embodiment is not necessarily short, as the beamsmay be of any length. However, it may be viewed as the shortestconfiguration for any given cell size.

In a second embodiment, the prosthesis may have curved beams like leafsprings which are repeatedly merged to alternating adjacent beamsthroughout their length. This second embodiment may also be viewed asthe single repetitive pattern of the first embodiment repeatedthroughout the length of the prosthesis. For example, a prosthesis maybe comprised of two or more of the single pattern prosthesis connectedend to end. Instead of actually connecting the prosthesis, they may beformed as a single structure. Thereby, the beams could be viewed ascontinuous throughout the length of the prosthesis. The beams would thenhave many curved portions which bring them in connection withalternating adjacent beams at merge sections.

The prosthesis may also embody these curved beams forming individualcylindrical elements and connected together by separate elements. Thus,a variety of prosthesis may be formed by connecting differentcylindrical elements together with different connecting elements. Oneconfiguration includes cylindrical elements having curved beams whichare only merged to adjacent beams at their end points connected tocylindrical elements having curved beams which are repeatedly merged toalternating adjacent beams throughout their length. This provides aprosthesis having varying strength and flexibility throughout itslength.

In the compressed condition the prosthesis may be intraluminallyinserted and delivered within a corporeal lumen. Once delivered to thesite to be treated, the prosthesis may be expanded and imbedded into theinterior of the lumen. Various methods for intraluminally expandingprostheses are well-known in the art. Expansion due to spring forces isparticularly suited for this invention. The super-elastic properties ofNickel-Titanium alloys (for example Nitinol) allow a great amount ofexpansion and compression of structures without permanent deformation.Thus a prosthesis made of such material may be compressed into a verysmall configuration, and will spring back into a preset form whenreleased. Other known methods of expansion include balloon expansion,and expansion due to the highly elastic properties of certain alloys.

The present invention may also be balloon expandable. To expand theprosthesis by balloon an angioplasty-type dilation catheter is insertedthrough a compressed or not-fully expanded prosthesis until the balloonportion of the catheter is longitudinally aligned within the prosthesis.The balloon is then expanded forcing the prosthesis radially outwardly.

Once expanded the prosthesis remains in the expanded condition, and thestrength of the prosthesis resists radial collapse. When used alone theprosthesis can expand and resist re-collapse of a previously collapsedor stenosed corporeal lumen. When used in combination with a graft, theprosthesis can maintain the graft open and secure the graft to thevessel.

Additional preferred embodiments of the present invention may providebenefits for high-expansion ratios. That is, the prosthesis may beconfigured to readily withstand high degrees of expansion andcompression. Prostheses having loop or eyelet connectors according tothis invention may also include beams of different lengths. Alternatingpairs of beams having longer lengths and shorter lengths provide a morecontrolled expansion. This configuration also permits the eyelets of theshorter length beams to nestle below the eyelets of the longer lengthbeams upon compression. A further feature aiding the expansion of theprosthesis includes varying the widths of the individual beams. Forexample, configuring beams having longer lengths with greater widthswill improve the prosthesis ability to accomplish high expansion.Furthermore, varying the width along the length of the beam may alsoimprove the expansion and compression abilities of prosthesis.

Other configurations of the prosthesis may be beneficial when theprosthesis is used in combination with a graft. The ends of prosthesiswhich are to be configured within a graft may include a flattenedbulbous tail. Such an extension of the prosthesis prevents wear on thefabric of the graft. Eyelets on the ends of the prosthesis may also beused for stitching the prosthesis together with the graft. Eyeletsprovide a good anchoring point for such stitching. Various combinationsof connected prosthesis according to the present invention may be usedwithin grafts.

In another aspect, the invention relates to a prosthesis or stent havinga plurality of cells. Each cell has a bottom end and a top end. Aflattened bulbous tail is at the bottom end of at least more than one ofthe cells. A flattened bulbous tail may also be at the top end of atleast more than one of the cells. Alternatively, the top end of eachcell may include an apex having a smaller surface area than theflattened bulbous tail. To facilitate compression of the stent fordelivery, adjacent flattened bulbous tails may be staggeredlongitudinally and the flattened bulbous tails may contour into the bodyof the stent. The circumference of the bottom and top ends of the stentmay also include a rounded or chamferred edge.

In a further aspect, the invention relates to a method of manufacturinga stent from a flat member that includes a first surface and a secondsurface. Material is removed from the flat member such that theremaining material forms a pattern of a circular array of cells of adesired stent pattern. A mandrel having an outside surface with a shapeis placed at the center of the pattern on the first surface of the flatmember and the flat member is formed around the mandrel, thereby causingthe flat member to assume a seamless tubular shape and becoming atubular member.

The flat member may include a Nitinol sheet. The removal step mayinclude chemical etching, laser cutting, electrical discharge machining,water-jet cutting, or stamping the stent pattern from the flat member.The outside surface of the mandrel may include a cylindrical shapehaving a diameter within the range of 20 mm to 34 mm. The forming stepmay include placing a collar on the second side of the flat memberaround the center of the pattern. The collar includes an inner surfaceand an outer surface with the inner surface having the same shape as themandrel, but being larger than the outside surface of the mandrel. Thesize difference between the outside surface of the mandrel and theinside surface of the collar is about the same as the thickness of theflat member. The forming step may further include providing relativemovement between the mandrel and the collar so that the collar causesthe flat member to surround the outside surface of the mandrel, therebycausing the flat member to become the tubular member. The method mayfurther include the step of heat treating the tubular member. Thetubular member may be heat treated to a temperature of around 280° C.for a duration of around three minutes, then cooled immediately. Theheat treating step may be performed while the tubular stent it is housedbetween the mandrel and the collar.

These and other advantages of the invention will become more apparentfrom the following detailed description of the preferred embodiments.When taken in conjunction with the accompanying exemplary drawings, theperson of skill in the art will appreciate that various embodimentsincorporate the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a first embodiment of the prosthesis inan expanded condition;

FIG. 2 is a perspective view of the first embodiment of the prosthesisin a compressed condition;

FIG. 3 is a cross-sectional view of the first embodiment of theprosthesis in an expanded condition;

FIG. 4 is a cross-sectional view of the first embodiment of theprosthesis in a compressed condition;

FIG. 5 is a perspective view of a second embodiment of the prosthesis;

FIG. 6 is a side view of a third embodiment of the prosthesis;

FIG. 7 is a top view of a portion of a flat pattern for the prosthesis;

FIG. 8 is a side view of a fourth embodiment of the prosthesis;

FIG. 9 is a side view of a vascular graft secured in a corporeal lumenby a prosthesis;

FIG. 10 is a side view of a prosthesis embedded in a corporeal lumen;

FIG. 11 is a perspective view of a thick-walled cylindrical tube withcells designed therein;

FIG. 12 is a flat pattern view of a portion of a prosthesis embodyingvariable thickness beams;

FIG. 13 is a flat pattern view of a portion of a prosthesis includingalternative embodiments of variable thickness beams;

FIG. 14 is a flat pattern view of a portion of a prosthesis includingadditional alternative embodiments of variable thickness beams;

FIG. 15 is a flat pattern view of a portion of a prosthesis includingadditional alternative embodiments having varying flexibility;

FIGS. 16 a and 16 b are side views of a first alternative embodiment ofthe beam ends and connector;

FIGS. 17 a and 17 b are side views of a second alternative embodiment ofthe beam ends and connector;

FIG. 18 is a flat pattern view of a first embodiment of a prosthesishaving the alternative beam end connectors shown in FIGS. 16 a and 16 b;

FIG. 19 is a flat pattern view of a second embodiment of a prosthesishaving the alternative beam end connectors shown in FIGS. 16 a and 16 b;

FIG. 20 is a flat pattern view of the prosthesis of FIG. 19 in acollapsed state;

FIG. 21 is a flat pattern view of a prosthesis from within a vasculargraft;

FIG. 22 a is a flat pattern view of a prosthesis having a flattenedbulbous tail at a bottom end of each of the cells of the prosthesis;

FIG. 22 b is a flat pattern view of a prosthesis having a flattenedbulbous tail at the bottom end and the top end of each of the cells;

FIG. 22 c is a cross-sectional view depicting contact points between theflattened bulbous tails of a prosthesis and a corporeal lumen wall;

FIG. 22 d is a cross-sectional view depicting an end of the prosthesisof FIG. 22 a having a radius along an edge;

FIG. 22 e is a cross-sectional view depicting an end of the prosthesisof FIG. 22 a having a chamfer along an edge;

FIG. 22 f is a plan view depicting a tubular structure having atraumaticends;

FIG. 23 a is a perspective view of a flat pattern layout for a stentdesign;

FIG. 23 b is a perspective view of the flat pattern layout of the stentof FIG. 23 a positioned with a mandrel and a collar prior to forming ofthe stent into a tubular shape;

FIG. 23 c is an elevation view of the flat pattern layout of the stentof FIG. 23 a positioned with the mandrel and collar of FIG. 23 b priorto forming of the stent into a tubular shape; and

FIG. 23 d is an elevation view of the stent of FIG. 23 a formed into thetubular shape and positioned between an interior surface of the collarand an exterior surface of the mandrel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description, as well as the Figures, describe embodimentsof the invention. These embodiments are exemplary of the inventors knownuses of the invention, and are not intended to limit the scope of theclaimed invention. Those skilled in the art of endoluminal devices willappreciate that the invention described herein may encompass manyembodiments.

As shown in the Figures, the present invention relates to an endoluminalprosthesis. More particularly, the invention is an expandable andcompressible prosthesis for repairing corporeal lumens. The prosthesismay be formed from a metallic cylinder by removal of cells. Theinvention also discloses a prosthesis with a flattened bulbous tail. Theinvention further discloses a stent being formed from a flat sheet ofmaterial.

As depicted in FIGS. 1 and 2, the result of removing cells 20 from themetallic cylinder 22 is a prosthesis 24 having a series of curved beams26 and merge sections 28. It is to be recognized that the prosthesis 24shown in FIG. 2 can be compressed, where desired, to a smaller diametersuch that the cells 20 are essentially defined by slits (not shown).

The beams 26 are generally longitudinal members circumferentially spacedabout the prosthesis 24. In one embodiment, as depicted in FIG. 5, theends of these beams 26 merge with the ends of circumferentially adjacentbeams to form the merge sections 28 only at the ends of the prosthesis.The ends of each beam 26 merge with one adjacent beam on the forward end30 and the opposite adjacent beam on the rear end 32. This creates asingle circumferential pattern of staggered half-cells 21 divided bybeams 26. (For comparison, a full cell 25 is identified in FIG. 1).Preferably, the beams 26 each include at least two curved segments 34 ofopposite orientations and an inflection point 36 near the mid-point ofthe beams.

In the embodiment depicted in FIG. 5, the merge sections 28 includeeither the forward ends 30 or rear ends 32 of two such beams 26 as wellas the ends of the prosthesis. These merge sections 28 are alsocircumferentially spaced about the prosthesis 24, preferablyequidistantly.

The single pattern as depicted in FIG. 5 may be extended to build longerprostheses 24. This may be done by extending the length of each beam 26with additional curved segments 34 and forming additional merge sections28. This forms the prosthesis 24 as depicted in FIGS. 1 and 2.Alternatively, multiple single patterns may be connected with a separateconnector element 58. The connector elements 58 may have variousconfigurations and be distributed throughout the prosthesis 24 in avariety of arrangements. Such a prosthesis 24, having “S”-shapedconnector elements at each merge section is depicted in FIG. 6. Otherembodiments (not shown) may have connectors with other shapes only atevery second or third merge section. Such embodiments may haveadvantages in providing longitudinal flexibility to the prosthesis 24.

In the embodiments depicted in FIGS. 1 and 2 the beams 26 may extendbeyond the first merge sections 28 to form additional merge sections 28.This configuration may also be viewed as the merge sections 28 connectedend-to-end with opposite facing merge sections 28. This may provide fora prosthesis 24 of greater lengths. The continuous beams 26 of thisembodiment merge with adjacent beams 26 repeatedly and alternatelythroughout their length. The continuous beams are comprised of multiplecurved segments 34. The merge sections 28 may also contain flat segments(not shown).

As depicted in FIGS. 3 and 4 the prosthesis 24 is preferably formed froma thick-walled cylinder 22. The difference between the external radiusof the cylinder and the internal radius of the cylinder defines a radialthickness 40. Preferably, the cells 20 of the prosthesis 24 are removedsuch that the remaining beams 26 have a width (measuredcircumferentially) 42 that is less than the radial thickness 40. Atypical design might have dimensions of 0.007″ circumferential width 42and 0.014″ radial thickness 40. This defines a deep cross-section forthe beams 26. To take advantage of the benefits of this invention, theradial thickness 40 of the beams 26 needs to be substantially greaterthan the circumferential width 42. Preferably, the radial thickness 40will be at least one and one-third (1⅓) times the circumferential width42.

A theoretical flat pattern of the beams 26 and merge sections 28, asdepicted in FIG. 7 reveals the novel configuration of the beams.Preferably, each beam 26 is continuously curved, alternating betweencurves 56 of opposite orientations throughout its length. In this idealconfiguration, the beams 26 form inflection points 36 between theopposite facing curves 56.

Each curve 56 in each beam 26 is defined by two radii, an internalradius 64 and an external radius 66. The difference between these radiidefine the circumferential width 42 of the beam 26.

The continuously curved configuration of beams 26 disposedlongitudinally along a cylinder, provides some of the unique propertiesof this invention. As the cylinder is expanded from a partiallycompressed configuration 50, the radii of each curve 56 within each beam26 becomes smaller as the beams spread apart. Since the beams 26 areideally continuously curved, the bending is spread throughout the entirelength of the beam 26. This spreads the resultant stresses throughoutthe beam 26 and reduces or minimizes stress concentrations.

As depicted in FIGS. 2 and 4 the use of deep cross-sections hassignificant advantages for collapsing the prosthesis 24 in preparationof intraluminal delivery. The deep cross-section allows for significantcompression without incidental overlapping of the beams 26. The largeradial thickness 40 of the beams 26 prevents one beam from extendingover the top of another.

As depicted in FIGS. 1 and 3 there are also advantages to the use ofdeep cross-sections in expansion of the prosthesis 24. In general, as acylindrical, expandable prosthesis is expanded, longitudinally-orientedmembers of the collapsed prosthesis tend to bend circumferentially. Therelatively narrow width of the beams 26 of the present prosthesis 24permits circumferential bending without inducing high stressconcentrations. The large overall cross-sectional area of the beams 26prevents re-compression of the prosthesis 24. The configuration of thecurved segments 34 spreads the stresses induced by expansion across theentire length of the beams 26, also reducing stress concentrations. In apreferred embodiment, the prosthesis is self-expandable. Alternatively,the prosthesis may be expanded by balloon.

FIGS. 1 and 3 and FIGS. 2 and 4 depict two separate configurations ofthe prosthesis 24. The prosthesis 24 of the present invention has anexpanded configuration 44 while deployed in the lumen as depicted inFIGS. 1 and 3. This configuration has a large inner diameter whichallows maximum patency of the lumen 46 to be treated. The prosthesis 24of the present invention also has a second, partially compressedconfiguration 50 as depicted in FIGS. 2 and 4. This configuration isbeneficial to the intraluminal delivery of the device which isfacilitated by a smaller external diameter.

In a typical procedure, the prosthesis 24 will be constrained in thecompressed configuration 50 within a catheter. The catheter may then beinserted into a small diameter lumen 46, such as the femoral artery. Toprevent damage to such an artery the entire system of catheter andprosthesis 24 must have as small a diameter as possible. Small diametersalso facilitate the navigation of the prosthesis 24 and catheter througharduous vasculature. Once inserted into such an artery, the catheter andprosthesis 24 may be advanced through the corporeal lumens, possibly tolarger arteries for treatment. The prosthesis 24 may then be releasedfrom the catheter. Spring forces within the compressed prosthesis of aself-expanding version will force the prosthesis from the partiallycompressed configuration 50 into the expanded configuration 44. In apreferred embodiment, the spring forces are great enough to expand thelumen of the diseased vessel as the prosthesis 24 expands. These forcesare also great enough to impinge the beams 26 into the tissues of thevessel. This impinging secures the prosthesis 24, and possibly anassociated graft 52, into place.

Another embodiment of the prosthesis 24, depicted in FIG. 8, has aconical rather than cylindrical shape while in the expandedconfiguration 44. In this embodiment, the prosthesis 24 has acylindrical shape in the compressed configuration 50. Upon expansion,however, a broader end 60 of the prosthesis 24 expands more than anarrower end 62. This conical embodiment of the prosthesis 24 is usefulin similarly shaped lumens and various configurations of grafts. Thebroader end 60 may include cells 20 that are longer and wider in theexpanded configuration 44 than those at the narrower end 62.

The prosthesis 24 of the current invention may be used in a variety ofprocedures, two of which are depicted in FIGS. 9 and 10. As depicted inFIG. 9, one or more prostheses embodying the present invention may beused in the treatment of aneurysms. An aneurysm is a weakening of thevessel wall of a vein or artery which causes a sack, or possibly arupture, to form in the lumen 46. When an aneurysm forms in theabdominal aorta, the condition can be life-threatening. A knowntreatment for aneurysms is the intraluminal delivery and implantation ofa vascular graft 52. Such a graft 52 bypasses the sack formed by theaneurysm and isolates the weakened tissues from the blood flow. Tooperate properly, the graft 52 must have leak-proof fixation to thehealthy vascular tissue on either side of the aneurysm. The prosthesis24 described herein may provide that fixation at one or more ends of thegraft 52. The prosthesis 24 may also extend throughout the length of thegraft 52. When expanded, the prosthesis 24 may compress the flexiblegraft material 52 against the arterial wall. Preferably, the prosthesis24 extends further from the aneurysm than the graft 52 so that parts ofthe prosthesis 24 are imbedded in healthy tissue. This configurationmaintains the patency of the artificial lumen of the graft 52 as well assecuring the graft in place by forcing the end of the graft against thewall of the lumen 46. The prosthesis 24 also ensures a leak-proof seal.

As depicted in FIG. 10, a prosthesis 24 embodying the present inventionmay be used to treat a stenosis or collapse of the lumen 46. Stenosis isoften caused by the gradual occlusion of veins or arteries through thebuild-up of plaque. Preferably a single prosthesis 24 is inserted intothe diseased vessel while mounted within a catheter. When the prosthesis24 is at the location of the narrowing, the prosthesis 24 may beexpanded. As depicted in FIG. 10, the spring forces of the prosthesisare preferably sufficient to expand the narrowed vessel. The prosthesis24 is thereby forced into the tissues of the lumen 46 to secure theprosthesis 24 in place. The structure of the prosthesis 24 resistscollapse after expansion.

The prosthesis 24 may be manufactured in the compressed configuration50, as in FIG. 2, or in the expanded configuration 44, as in FIG. 1, orin any configuration in between. The manufacturing procedure requiresthe removal of cells 20 from a thick-walled cylinder 22. This may beaccomplished with several known manufacturing methods, such as lasercutting, chemical etching, photo-etching, electrical discharge machining(EDM) and mechanical means. Two materials found to be particularlysuited to this application are implantable stainless steel andNickel-Titanium alloys, such as Nitinol.

As depicted in FIG. 11, each cell 20 of the endoprosthesis 24 mayconsist of two sides 54 having three curves 56 and two inflection points36. Such a configuration produces almond-shaped cells. There may also beflat portions (not shown) designed into the cell 20. These cells 20 aredesigned on the thick walled cylinder 22 in a pattern which repeatsalong the length of the cylinder 22. This pattern is repeated with alongitudinal stagger of half a cell 20 around the circumference of thecylinder 22. The pattern also includes half cells at each end of thetube. Upon removal of the cells 20, the remaining material constitutesthe prosthesis 24 described herein.

The prosthesis 24 may be formed from a thick walled cylinder 22approximately the size of the compressed configuration 50. This thickwalled cylinder 22 may be a Nickel Titanium alloy. Cells 20 are lasercut into the thick walled cylinder 22 while the thick walled cylinder 22is mounted over a wire. The cells 20 are formed in a long, narrowconfiguration with each of the curves 56 having large radii.

After the cells 20 are cut into the thick walled cylinder 22, theprosthesis 24 is cleaned and deburred to eliminate manufacturingirregularities. This may include blasting techniques, acid etching,ultrasonic cleaning and/or other well known methods of cleaning.

The prosthesis may then be stretched into more expanded configurations.One method of expanding the prosthesis is by mechanically stretching itover a mandrel. The mandrel may be specifically designed with pins tomaintain the desired curvature of the beams. Once stretched, theprosthesis is annealed to set the new expanded shape of the prosthesis.Annealing can be accomplished by heating the prosthesis within a varietyof media, such as air, molten salt, inert gas or vacuum. Annealing at260-288° C. is appropriate for Nickel-Titanium alloys. After stretching,the prosthesis 24 is cleaned again. This process of stretching,annealing and cleaning can be repeated until the desired configurationis obtained. Once the desired configuration has been obtained, theprosthesis is electropolished by any of the well-known methods.

Alternatively, a prosthesis 24 may be formed from a Nickel Titaniumthick walled cylinder 22 approximately the size of the expandedconfiguration. In this process, cells 20 are cut into the thick walledcylinder in a shorter and wider configuration. This method wouldeliminate the need to stretch and anneal the prosthesis 24 to achievethe expanded configuration 94.

As best seen in FIGS. 12-14, it is also contemplated that the beams of aprosthesis may embody variable width beams or struts 70 and generallyuniform width beams or struts 71. The incorporation of variable widthstruts 70 into a prosthesis facilitates uniform expansion. For example,to achieve uniform expansion, it is desirable to have struts 70 of thesame width meeting at connecting junctions 72. Asymmetric prosthesisportions 74, 76, as shown in FIGS. 12 and 13, may further require thestrut 70 to embody a width that gradually varies along the length of thestrut 70. Moreover, as shown in FIG. 14, where a prosthesis portion 78embodies a plurality of adjacent oriented cells 80, the point ofconnection 82, 83 between adjacent cells 80 may be varied in length, forexample to accommodate a hole 84. To facilitate uniform expansion ofsuch a prosthesis portion 78, the struts 70 extending from a relativelyshorter point of connection 82 between adjacent cells 80 can embody atapering thickness.

The novel features of the present invention may be applied to configurea prosthesis having variable properties throughout the length of theprosthesis. As an example, and as depicted in FIG. 15, the flexibilityof the prosthesis 24 may vary along the length of the prosthesis. Toaccomplish this, connector elements 58 may be used to combine segments90 of the prosthesis 24. Each segment 90 may be composed of curved beams26 in the various configurations described above. In a preferredembodiment, a segment 90 composed of full cells 20 may be combined withmultiple segments 90 composed of half-cells 21. The portion of theprosthesis 24 composed of half-cell 21 segments 90 will tend to besignificantly more flexible longitudinally and slightly more flexibleradially. The invention includes any combination of full cells andhalf-cells in a prosthesis including, but not limited to, full cellsbetween half-cells.

One application of a prosthesis 24 having variable flexibilitythroughout its length is for the support of a vascular graft 52. In suchan application, the more flexible segments 90 of the prosthesis 24 maybe configured to support the artificial lumen 46 of the graft 52. Theless flexible segments 90 of the prosthesis 24 may be configured toextend beyond the artificial lumen 46 and into the patients naturallumen. In this manner, the less flexible segments 90 help secure thegraft 52 into place, while the more flexible segments 90 support thematerial of the graft 52.

The invention described herein may also embody features to facilitatethe high ratios of expansion possible with the prosthesis 24. Asdepicted in FIGS. 16 a and 16 b, as well as in FIGS. 17 a and 17 b, theends 92 of the beams 26 may be connected in a manner which evenlydistributes the stresses incurred by expansion and compression.

An eyelet or loop connector 94 (shown in FIGS. 16 a and 16 b) mayconnect the ends of the beams 26. These eyelet connectors 94 distributethe stresses created by compression of the prosthesis 94. As theprosthesis 24 is initially compressed and adjacent beams 26 are broughttogether, the bending and resultant stresses are initially concentratedin the eyelet or loop portion of the connector 94. Eventually, a contactarea 96 is formed at the edge of the eyelet connector 24. As theprosthesis 24 is further compressed and adjacent beams 26 are broughteven closer together, the bending and resulting stresses areconcentrated at the ends of the beams 26 near the contact area 96. Evenfurther compression may relieve the stresses in the eyelet connector 24by creating a fulcrum at the contact area 96.

A prosthesis 24 composed entirely of eyelet connectors 94, as depictedin FIG. 18, may facilitate the distribution of stresses induced by highexpansion ratios. Thus, the prosthesis 24 of the present invention maybe used in particularly large corporeal lumens, such as the abdominalaorta. This same prosthesis 24 may also be introduced into relativelysmall corporeal lumens, such as the femoral artery. Such an applicationrequires the prosthesis to transition between a highly compressed statefor insertion into the femoral artery, to a highly expanded state forimplantation into the abdominal aorta. This application, as well asothers, induce high stresses on the prosthesis 24 through bending of thebeams 26 in expansion and compression.

A similar distribution of the stresses may be accomplished byconfiguring the ends 92 of the beams 26 into increased contact endconnectors 98 (FIGS. 17 a and 17 b). In such a configuration (depictedin FIGS. 17 a and 17 b), the ends 92 of the beam 26 connect togetherwith a substantial area of contact 96 near the actual connection. As theprosthesis 24 is compressed and the beams 26 are brought closertogether, the stresses due to bending are concentrated in the beams 26near the contact area 96. The contact area 96 expands as the beams 26are brought closer together and the stress concentrations are therebydistributed along the length of the beams 26.

To further support high expansion ratios, the prosthesis 24 of thepresent invention may be configured to pack tightly for compression intoa collapsed state. One example, as depicted in FIGS. 19 and 20 mayutilize eyelet connectors 94 aligned to differing heights. That is,every other eyelet connector 94 may be configured upon beams 26 of afirst, greater length 100, while each other eyelet connector 94 may beconfigured upon beams 26 of a second, lesser length 102. Thus the eyeletconnectors 94 configured upon beams at the second length would havetheir greatest width at the same location that the eyelet connectors 94configured upon beams at the first length have their least width. Inthis manner, the beams 26 and eyelet connectors 94 fit together in themost compact condition while compressed. Similar results may beaccomplished by varying the thickness of the beams 26 and connectors 94.

Varying the circumferential width 42 of the beams 26 may also providebenefits in high expansion ratios. For example, using larger widths onbeams 26 of a first, greater length 100 may help control the expansionof the prosthesis 24 and reduce stress concentrations. Varying thecircumferential width 42 along the length of individual beams 26 mayprovide superior nesting when the prosthesis is provided with eyeletconnectors 94.

Further configurations, as depicted in FIG. 21, may be advantageous whenthe prosthesis 24 is configured for use in a vascular graft 52. Eyeletconnectors 94 may be used to provide an anchor for the stitching 104between the graft 52 and the prosthesis 24. The merge sections 28 at theend of the prosthesis may include a flattened bulbous tail 106. Thetails 106 reduce the wearing on the fabric of the graft 52. Tails 106may also help control the expansion of the prosthesis 24. Instead ofspringing open when the ends of the prosthesis 24 are released, thetails 106 may remain constrained within a delivery catheter and providethe prosthesis 24 with a slower, more controlled expansion.

As depicted in FIG. 22 a, an alternative configuration of a prosthesisor stent 120 for use in a vascular graft assembly includes a flattenedbulbous tail 122 at a bottom 124 end of at least more than one cell 126of the stent. In one embodiment, the bottom 124 end of each cell 126includes a flattened bulbous tail 122. Adjacent tails 122 are staggeredlongitudinally, thus allowing each tail to attain a maximum size withoutinterfering with compression of the stent 120 for packing of the stentinto a delivery catheter. The flattened bulbous tails 122 include largersurface areas than the apices 128 at the top 130 of the stent 120.

While a vascular graft assembly typically includes at least one fixationstent that attaches a tubular graft to a corporeal lumen, the flattenedbulbous tail stent 120 of the present invention aids in sealing thegraft to the corporeal lumen. In one embodiment, the tubular graft ofthe vascular graft assembly includes a first end region and a second endregion. The stent 120 may be located within the first end region of thegraft with the flattened bulbous tails 122 of the bottom 124 end of thestent positioned inside the tubular graft and the apices 128 at the top130 end of the stent located beyond the first end region of the graft.Such placement of the stent 120 positions the apices 128 of the stentexternal to the graft.

Anytime a stent or prosthesis is deployed in a graft, there is anopportunity for wear. The flattened bulbous tail 122 of the stent 120reduces the amount of wear between the stent and the softer material ofthe vascular grafts. Prosthesis or stent to graft wear is a function ofthe radial force per unit area of the stent, the sharpness of the stent,and the amount of relative motion between the stent and the graft. Inthe present invention, the relatively large surface area of theflattened bulbous tail 122 displaces the radial force of the tail over alarger area, thereby effectively reducing the radial force per unit areaof the stent, and the sharpness of the stent 120 is decreased by thecircular shape of the flattened bulbous tail. By reducing the radialforce per unit area and decreasing the sharpness through the use of theflattened bulbous tail, the amount of graft wear for a given stent overa given time is reduced, thereby prolonging the useful life of thegraft. The flattened bulbous tail 122 configuration disclosed herein mayalso be applied to the previously disclosed prostheses and stents.

Referring to FIG. 22 b, flattened bulbous tails 122 may replace theapices at the top 130 end of at least more than one cell 126 of thestent 120 or prosthesis in order to minimize wear at the contact pointbetween a stent and a corporeal lumen wall 132 (FIG. 22 c). In oneembodiment, the top 130 end of each of the cells 126 and the bottom 124end of each of the cells within the stent 120 includes a flattenedbulbous tail 122. FIG. 22 c depicts the contact points between oneembodiment of a stent 120 having flattened bulbous tails 122 and thecorporeal lumen wall 132. FIG. 22 b depicts the flattened bulbous tails122 at the bottom 124 end and top 130 end of the cells 126 offsetlongitudinally such that only one half to the flattened bulbous tailsoccupy the same axial location when the stent 120 is compressed fordelivery.

An alternate method of minimizing wear at the contact point between astent 120 and the corporeal lumen wall 132 or graft material is to grinda radius 134 (FIG. 22 d) or chamfer 136 (FIG. 22 e) around thecircumference of the bottom 124 end and the top 130 end of each of thecells 126. Alternatively, if the stent 120 is laser cut from a Nitinoltube 138 (FIG. 22 f) the bottom 124 end and top 130 end of the tube maybe ground prior to cutting the cell 126 pattern within the stent. With aradius 134 ground around the circumference of the bottom 124 or top 130end of each of the cells 126, the contact point between the stent 120and the vessel lumen wall 132 or graft material is the tangent point ofthe radius.

A method for fabricating stents from a flat sheet of material isdepicted in FIGS. 23 a-23 d. The general geometry of the stent pattern140 (FIG. 23 a) can be cut directly from a workpiece 142 of asubstantially flat piece of material. In a preferred embodiment, theworkpiece 142 includes a shape memory sheet metal material, such as aNitinol sheet. Cutting the general geometry of the stent pattern may beaccomplished by any of several known manufacturing methods, such aslaser cutting, chemical etching, photo-etching, electric-dischargemachining (EDM), water-jet cutting, stamping, and other mechanicalmeans.

Material removal, such as the cutting of the stent pattern 140, iseasier to perform on a flat piece of material than on a tubular piece ofmaterial. For instance, when cutting a stent pattern from a tubularpiece of material, it is often necessary to place a secondary fillermaterial into the lumen of the tube to prevent damage to one side of thetube while an opposite side of the tube is being cut. Also, it is oftennecessary to rotate a tubular piece of material about its axis while astent pattern is being cut into it. While these steps may add to thecomplexity of cutting a stent pattern into a piece of material, they mayalso add to the cost to cut the stent pattern. Further, the cost of rawtubular material is often higher than the cost of sheet material.Therefore, producing a tubular stent from a flat sheet of material mayreduce the cost of producing the stent.

The stent pattern 140 may include a circular array of cells 143. Afterthe stent pattern 140 is produced, a cylindrical mandrel 144 may beplaced at the center 146 of the pattern 140 within the workpiece 142from a first side of the flat sheet and a collar 148 may be placedaround the center of the pattern on a second side of the flat sheet.Relative movement of the collar 148 and the mandrel 144 toward eachother causes the flat workpiece 142 to form around the mandrel and toassume a seamless tubular shape 150 (FIG. 23 d) between the mandrel andthe collar. Although a cylindrical-shape mandrel 144 and collar 148 aredisclosed, other shapes may also be used, such as a conical shape.

In a preferred embodiment, the outer diameter of the mandrel 144 rangesfrom 20 mm to 34 mm, depending on the size of the stent 152 to beproduced, however a smaller or larger diameter can be used. The mandrel144 can also incorporate grooves (not shown) or pins (not shown) whichcoincide with the cells 143 to control or change the shape of the stent152. The inside surface 154 of the collar 148 may have the same shape asthe outside surface 156 of the mandrel 144, but with the collar in placeover the mandrel there is a gap 160 all around about the same size asthe thickness of the stent 152.

The stent 152 may be heat treated, such as in a furnace (not shown) orin a salt pot (not shown), while housed between the mandrel 144 and thecollar 148. During the heat treatment process, the material may beheated to a temperature of around 280° C. (535° F.) for a duration ofabout three minutes, and then cooled rapidly to set the shape into thematerial. The method disclosed herein for fabricating stents from a flatsheet of material may also be applied to the previously disclosedprostheses and stents.

While the present invention has been described herein in terms of aprosthesis or stent for the repair of blood vessels, those of skill inthe art will readily recognize that prostheses embodying the describedinvention can be used to treat a variety of corporeal lumens, forexample the bronchial tree or intestines. The invention described hereinis intended to be limited only by the claims that follow and not by anyparticular embodiment.

1. A prosthesis, comprising: a plurality of variable width beams eachhaving a length, a circumferential width which varies along the lengthof the beam and a radial thickness; a plurality of uniform width beamseach having a length, a circumferential width which is substantially thesame along the length of the beam and a radial thickness, wherein theplurality of variable width beams and uniform width beams cooperate toform a structural body having a first end and a second end which definesa longitudinal axis, the structural body having a collapsed position andan expanded position; and a plurality of connecting junctions, eachconnecting junction connecting an end of a uniform width beam to the endof a variable width beam such that the uniform width beam is locatedlaterally adjacent along the longitudinal axis to the variable widthbeam to which it connected, wherein the variable width beams and uniformwidth beams have substantially the same radial thickness and the radialthickness is greater than the largest circumferential width of eitherthe variable width beams or the uniform width beams.
 2. The prosthesisof claim 1 wherein the lengths of the variable width beams are less thanthe lengths of the uniform length beams.
 3. The prosthesis of claim 2wherein the lengths of the variable width beams are substantially thesame.
 4. The prosthesis of claim 3 wherein the lengths of the uniformwidth beams are substantially the same.
 5. The prosthesis of claim 1wherein the width of the uniform width beam is substantially the samewidth as the variable width beam at the connecting junction.
 6. Theprosthesis of claim 5 wherein the width of the variable width beamdecreases along its length away from the connecting junction.
 7. Theprosthesis of claim 6 wherein a second variable width beam is attachedto the first mentioned variable width beam at a second connectingjunction.
 8. The prosthesis of claim 7 wherein the width of the firstmentioned variable width beam and the width of the second mentionedvariable width beam at the second connecting junction are at theirsmallest.
 9. The prosthesis of claim 8 wherein the length of the uniformwidth beams is larger than the length of the variable width beams. 10.The prosthesis of claim 1 wherein each uniform width beam and variablewidth beam has a first end and a second end, each of the first ends andsecond ends of the uniform width beam being attached to either a firstor second end of a variable width beam at a connecting junction.
 11. Theprosthesis of claim 10 wherein each variable width beam has a first orsecond end attached to a first or second end of another variable widthbeam at a connecting junction.
 12. The prosthesis of claim 11 whereinthe length of the uniform width beams is larger than the length of thevariable width beams.
 13. The prosthesis of claim 11 wherein the widthof the variable width beam at the connecting junction with the uniformwidth beam is larger than the width of the variable width beam at theconnecting junction with the variable width beam.
 14. The prosthesis ofclaim 10 wherein the width of the variable width beam decreases alongits length away from the connecting junction.
 15. The prosthesis ofclaim 14 wherein the width of the variable width beam and width of theuniform width beam are generally the same at the connecting junction.16. The prosthesis of claim 1 wherein a pair of variable width beams arelocated between a pair of uniform width beams.
 17. A prosthesis,comprising: a plurality of variable width beams, each variable widthbeam having a first end and a second end defining a length therebetween,a circumferential width which varies along the length of the beam and aradial thickness; a plurality of uniform width beams, each uniform widthbeam having a first end and a second end defining a length therebetween,a circumferential width which is substantially the same along the lengthof the beam and a radial thickness, wherein the plurality of variablewidth beams and uniform width beams cooperate to form a structural bodyhaving a first end and a second end, the structural body having acollapsed position and an expanded position, the first end of eachuniform width beam being attached to the first end of a variable widthbeam at a first connecting junction, the first connecting junctionsforming the first end of the structural body and the second end of eachuniform width beam being attached to the first end of a variable widthbeam at a second connecting junction, the second connecting junctionsforming the second end of the structural body.
 18. The prosthesis ofclaim 17 wherein the second end of each variable width beam is attachedto the second end of another variable width beam at a third connectingjunction.
 19. The prosthesis of claim 18 wherein each of the thirdconnecting junctions is disposed between the first end and the secondend of the structural body.
 20. The prosthesis of claim 17 wherein thelengths of the variable width beams are less than the lengths of theuniform length beams.
 21. The prosthesis of claim 20 wherein the lengthsof the variable width beams are substantially the same.
 22. Theprosthesis of claim 21 wherein the lengths of the uniform width beamsare substantially the same.
 23. The prosthesis of claim 17 wherein thevariable width beams and uniform width beams have substantially the sameradial thickness and the radial thickness is greater than the largestcircumferential width of either the variable width beams or the uniformwidth beams.
 24. The prosthesis of claim 17 wherein a pair of variablewidth beams are located between a pair of uniform width beams.